Abstract

The Lengshuiqing magmatic Ni-Cu ore deposit is hosted in four small mid-Neoproterozoic mafic-ultramafic intrusions in the western margin of the Yangtze craton in China. The ore-bearing intrusions were emplaced into early Neoproterozoic metamorphosed volcanic-sedimentary rocks, granites, and diorites. The intrusions are dominated by lherzolite, olivine websterite, websterite, and gabbro, with sulfide mineralization associated with ultramafic rocks in the lower parts of the intrusions. The forsterite contents of olivine crystals in the Lengshuiqing ore-bearing intrusions are up to 86 mol %, indicating that their parental magma experienced moderate degrees of fractional crystallization. Olivine crystals in the intrusions are depleted in Ca (<1,000 ppm), a common feature for arc-type mafic-ultramafic intrusions worldwide. Moreover, clinopyroxene and Cr spinel in the Lengshuiqing ore-bearing mafic-ultramafic intrusions are remarkably similar to those in arc-type olivine-clinopyroxene cumulates elsewhere in the world. Whole-rock samples from these intrusions are all characterized by moderate light rare earth element (REE) enrichments relative to heavy REEs, moderate to significant degrees of negative Nb-Ta anomalies ([Th/Nb]N from 1.5–5), positive εNd(t) from 1.1 to 5.8, and low (87Sr/86Sr)i from 0.7038 to 0.7050. The isotope data indicate that the parental magma for these intrusions experienced only minor degrees of crustal contamination that cannot fully account for the observed negative Nb-Ta anomalies in the rocks. The data imply that the parental magmas were already depleted in Nb and Ta prior to crustal contamination. Taken together, the data support the view of an arc setting for the Lengshuiqing ore-bearing mafic-ultramafic intrusions. The occurrence of rare rounded sulfide inclusions in some olivine and spinel crystals in the intrusions indicates that immiscible sulfide droplets were present during olivine and spinel crystallization. Like most other arc-type magmatic sulfide deposits, the platinum group element (PGE) tenors of the Lengshuiqing deposit are very low. The estimated initial contents of PGEs in the parental magma of the Lengshuiqing deposit are two orders of magnitude lower than those in PGE undepleted picrites of continental flood basalt provinces such as the Emeishan and Siberian traps. The PGE depletions in the parental magma of the Lengshuiqing deposit are likely due to previous sulfide segregation during magma ascent or sulfide retention in the source mantle during partial melting. The δ34S, γOs(t), and S/Se of the sulfide ores are from –4.0 to 1.3 ‰, from 115 to 320, and from 4,800 to 12,500, respectively. These values are significantly different from typical mantle values and indicate that addition of external sulfur played a major role in triggering sulfide saturation in the magma. The results from this study confirm that arc-type mafic-ultramafic intrusions are capable of hosting economically valuable magmatic Ni-Cu deposits and should be considered as part of global Ni exploration programs.

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